1. Field of the Invention
The present invention relates to a direct current power supply. More specifically, the present invention relates to a battery backup system for an alternating current powered direct current power supply.
2. Description of the Prior Art
Prior art approaches to battery backup systems for AC energized DC supplies have included a backup battery which is selectively switched to the input of a linear voltage regulator upon failure of an alternating current, or AC, source supplying the regulator, a battery continuously connected, or "floated", directly across the output of a direct current, or DC, power supply, a battery connected across the output of a direct current power supply with a diode coupling which is normally reversed bias by the output of the regulated power supply, and other similar battery backup schemes. The prior art battery backup system wherein the backup battery was used as the input to the regulator had the disadvantages of being dependent on the reliability of the regulator and involving high power dissipation during both battery and normal AC operation. The prior art backup system using a battery connected directly across the power supply output has the disadvantage that the AC supply must have very good regulation and must be large enough to handle the power for the load and to charge the battery at the same time. The prior art backup system having a battery coupled through a diode to the output line also requires a power supply with good regulation to keep the diode reversed biased under normal conditions. Accordingly, it is desirable to provide a battery backup unit that has high efficiency during both AC and battery operation as well as having a measure of independence from the operation of the regulated DC power supply normally supplying DC power to a load.